1. Field of the Invention
The invention relates to methods for tuning rotating mass structural stiffness and vibration excitation frequencies in dynamoelectric machines, and more particularly to methods for tuning rotor stiffness and critical speed vibration in induction motors. The invention enables a motor manufacturer to vary selectively rotor stiffness and the motor's critical vibration speed, preferably so that the critical speed is above the highest planned motor operational speed in a single or variable speed motor application.
2. Description of the Prior Art
Machines with rotating masses, including electrodynamic machines, inherently have critical vibration excitation speeds attributable to the structural mechanics of the device and associated system operational forces. In electromechanical systems it is advisable to avoid operating machinery at its first or subsequent harmonic vibration frequency so as to avoid premature wear and possibly damage to the machine. Customers for some intended motor applications establish purchase specifications that demand very low operational vibration throughout the range of all intended operating speeds. One way to avoid operating machinery at a critical vibration speed is to identify that speed and henceforth set the maximum permissible operating speed limit below the critical speed. It follows that a motor manufacturer should seek ways to increase the critical vibration speed of its products to be above prospective customer operational speed requirements, in order to meet customer vibration specifications.
Induction motor vibration influences that ultimately contribute to motor critical vibration speed include among other things: rotor length to diameter ratio, rotor core cross-sectional structure, shrink fit pressure between the rotor core and shaft, stacking spacing between adjoining rotor core laminas, changes in alternating current excitation frequency established by variable speed motor drive controllers, and oil whip rotordynamic stability induced by the hydrodynamic bearings that support the rotor shaft. With respect to excitation frequency vibration influences, induction motors generally are optimized for 50 Hz or 60 Hz alternating current excitation frequencies, including any operational vibration responses. However, AC induction motors that are coupled to variable speed motor control drives often vary the AC excitation frequencies in a range from 30 Hz to 75 Hz. This increased range of AC variable excitation frequencies increase proportionally the motor's responsive critical vibration frequency range. Oil whip is one form of responsive vibration resulting from shaft bearing natural excitation frequency being influenced by motor operational speed. Generally oil lubricated hydrodynamic bearings in induction motors are susceptible to oil whip when the rotor speed is over roughly twice bearing system's inherent critical speed. Oil whip vibration instability causes the rotor shaft to flex in a sinusoidal curve between the bearings.
Cumulatively the sum of all induction motor vibration influences and the motor's physical structure, including manufacturing variances, establish the motor critical vibration speed. Motor manufacturers want to design motors with critical vibration speeds above their customers' maximum operational speed, in order to satisfy customer demands for “low vibration” motors.
One known suggested potential solution for rotor vibration instability is to stiffen the assembled rotor rotating structure by stiffening the rotor shaft. In the example of oil whip induced vibration instability, stiffening the rotor shaft tends to raise the motor speed that would otherwise cause the onset of oil whip because there is less shaft flexure. Thus, increasing the rotor shaft stiffness raises the motor's critical vibration frequency. There are motor design performance tradeoffs associated with increasing rotor shaft diameter. Induction motor output performance is influenced by the relative proportional volumes of the stator, rotor and shaft within the motor housing. For example, increased shaft diameter increases rotor stiffness but reduces cross-sectional space available to provide for axial cooling passages through the rotor core. Conversely, reducing shaft diameter may enable a designer to improve motor cooling but risks increasing shaft flexure and lowering the motor critical vibration speed. It should also be noted that an optimal motor shaft diameter for one motor frame design in a 60 Hz alternating current system application may not be optimal for a 50 Hz AC application. Focusing too narrowly on one component's optimization (here shaft diameter) is not as preferable as a broader spectrum, holistic “system” solution.
Despite efforts to raise induction motor critical vibration speed through rotor stiffness and other design optimization, potential manufacturing variations alter critical vibration speed for any given completed physical motor. Manufacturing variations typically result from variations in rotor laminas stack compression and spacing from lamination to lamination, shrink fit relative force stresses locally within a specific rotor stack and from rotor to rotor, shaft straightness resulting from both fabrication and assembly handling variations and concentricity of the rotating rotor relative to the stator and motor housing. Variations in manufacturing tolerances and processes lead to different rotordynamic performances, and ultimately differing vibration critical speeds from motor to motor.
Unfortunately, known rotordynamic stability solutions to attempt to raise critical vibration speeds through changes in induction motor design have not provided system-level solutions of how to tune critical vibration characteristics from one motor design to another, from one application of a specific motor design to a different application, or individual manufacturing variances.
Thus, a need exists in the art for a holistic, generalized systematic approach for tuning rotordynamic stability, and hence tuning the critical vibration speed of any individual motor for any given motor design, intended application environment or factory fabrication/refitting variances.